Wear resistant austenitic steel having superior machinability and ductility, and method for producing same

ABSTRACT

There are provided a wear resistant austenitic steel having superior machinability and toughness in weld heat affected zones and a method for producing the austenitic steel. The austenitic steel includes, by weight %, manganese (Mn): 15% to 25%, carbon (C): 0.8% to 1.8%, copper (Cu) satisfying 0.7C-0.56(%)≦Cu≦5%, and the balance of iron (Fe) and inevitable impurities, wherein the weld heat affected zones have a Charpy impact value of 100 J or greater at −40° C. The toughness of the austenitic steel is not decreased in weld heat affected zones because the formation of carbides during welding is suppressed, and the machinability of the austenitic steel is improved so that a cutting process may be easily performed on the austenitic steel. The corrosion resistance of the austenitic steel is improved so that the austenitic steel may be used for an extended period of time in corrosive environments.

TECHNICAL FIELD

The present disclosure relates to wear resistant austenitic steel havingsuperior machinability and ductility, and a method for producing thewear resistant austenitic steel.

BACKGROUND ART

Along with the development of the mining, oil, and gas industries, thewear on steel used for mining, transportation, and refining applicationshas become problematic. Particularly, as oil sands have been recentlydeveloped in earnest as an unconventional source of petroleum, the wearon steel members caused by slurry containing oil, gravel, and sand isone of the main factors increasing the production cost of oil from oilsands, and thus, the development and practical implementation of steelhaving a high degree of resistance to wear are increasingly required.

In the mining industry, Hadfield steel having high wear resistance hasbeen mainly been used. Hadfield steel is high-strength steel having ahigh manganese content, and there have been steady efforts to improvethe wear resistance of such steel by adding large amounts of carbon andmanganese thereto to increase the formation of austenite and wearresistance therein. However, due to a high carbon content in Hadfieldsteel, carbides may be formed at high temperature in a network manneralong austenite grain boundaries of the Hadfield steel, and thus thephysical properties of the Hadfield steel (particularly, ductility) aremarkedly worsened.

To prevent the formation of such network-shaped precipitates ofcarbides, a method for manufacturing high-manganese steel by rapidlycooling the high-manganese steel to room temperature after a solutionheat treatment or a hot working process is performed on thehigh-manganese steel at a high temperature has been proposed. However,if a relatively thick steel sheet is formed by the proposed method, theeffect of preventing the precipitation of carbides may not besufficiently obtained by rapid cooling. In addition, if a weldingprocess is performed, it is difficult to control the rate of coolingafter the welding process and thus difficult to suppress the formationof network-shaped precipitates of carbides. Therefore, physicalproperties of steel may be markedly worsened. In addition, alloyingelements such as manganese or carbon inevitably segregate in ahigh-manganese ingot or slab during solidification, and such segregationis facilitated in a post processing process such as a hot rollingprocess. As a result, carbides may partially precipitate in the form ofa network along intensive segregation zones of a final product, and thusthe microstructure of the final product may be inhomogeneous, resultingin poor physical properties.

Generally, the content of carbon in steel may be increased to improvethe wear resistance of steel, and the content of manganese in the steelmay be increased to prevent the deterioration of physical properties ofthe steel caused by the precipitation of carbides. However, this methodincreases the amounts of alloying elements and thus the manufacturingcost of steel. Furthermore, the addition of manganese to steel decreasesthe corrosion resistance of the steel as compared with general carbonsteel. Thus, such steel may not be used in fields requiring corrosionresistant steel.

Furthermore, since the machinability of austenitic high-manganese steelis poor due to a high degree of work hardenability, the lifespans ofcutting tools may be decreased, and thus costs for cutting tools may beincreased. In addition, process suspension times may be increased due tothe need for the frequent replacement of cutting tools. Eventually,manufacturing costs may be increased.

DISCLOSURE Technical Problem

Aspects of the present disclosure may provide austenitic steel havingimproved machinability, ductility, and wear resistance throughsuppressing the formation of carbides, and a method for producing theaustenitic steel.

However, aspects of the present disclosure are not limited thereto.Additional aspects will be set forth in part in the description whichfollows, and will be apparent from the description to those havingordinary skill in the art to which the present disclosure pertains.

Technical Solution

According to an aspect of the present disclosure, wear resistantaustenitic steel having superior machinability and ductility mayinclude, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and thebalance of iron (Fe) and inevitable impurities.

According to another aspect of the present disclosure, a method forproducing wear resistant austenitic steel having superior machinabilityand ductility may include: reheating a steel slab to a temperature of1050° C. to 1250° C., the steel slab including, by weight %, 8% to 15%of manganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu)satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) andinevitable impurities; performing a finish hot rolling process on thesteel slab within a temperature range of 800° C. to 1050° C. to form asteel sheet; and cooling the hot-rolled steel sheet to a temperature of600° C. or lower at a cooling rate of 10° C./s to 100° C./s,

Advantageous Effects

According to the present disclosure, the formation of carbides in theaustenitic steel may be suppressed to prevent the deterioration of theaustenitic steel, and the wear resistance of the austenitic steel may besufficiently improved. Therefore, the austenitic steel may be used foran extended period of time, even in corrosive environments.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a relationship between manganese andcarbon according to an embodiment of the present disclosure.

FIG. 2 is a microstructure image of steel in an example of the presentdisclosure.

FIG. 3 is a graph illustrating a relationship between the content ofsulfur and machinability in an example of the present disclosure.

BEST MODE

Hereafter, wear resistant austenitic steel having superior machinabilityand ductility, and a method for producing the wear resistant austeniticsteel will be described in detail according to embodiments of thepresent disclosure, so that those of ordinary skill in the related artmay clearly understand the scope and spirit of the embodiments of thepresent disclosure.

The inventors found that if the composition of steel is properlyadjusted, the steel has a high degree of wear resistance without adecrease in ductility caused by carbides and a high degree ofmachinability. Based on this knowledge, the inventors invented wearresistant austenitic steel and a method of producing the wear resistantaustenitic steel.

That is, manganese and carbon are added to the steel of the embodimentsof the present disclosure to improve the wear resistance of the steelwhile controlling the content of the carbon relative to the content ofthe manganese to minimize the formation of carbides. Furthermore,additional elements are added to the steel to further suppress theformation of carbides and thus to sufficiently improve the toughness ofthe steel in addition to improving the wear resistance of the steel, andin conjunction therewith, the contents of calcium and sulfur in thesteel are adjusted to markedly improve the machinability of the steel(austenitic high-manganese steel).

According to the embodiments of the present disclosure, the steel mayinclude, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and thebalance of iron (Fe) and inevitable impurities.

The numerical ranges of the contents of the elements are set because ofreasons described below. In the following description, the content ofeach element is given in weight % unless otherwise specified.

Manganese (Mn): 8% to 15%

Manganese is a main element for stabilizing austenite in high manganesesteel like the steel of the embodiments of the present disclosure. Inthe embodiments of the present disclosure, it may be preferable that thecontent of manganese be 8% or greater for forming austenite as a maincomponent of the microstructure of the steel. If the content ofmanganese is less than 8%, ferrite may be formed, and thus austenite maynot be sufficiently formed. On the other hand, if the content ofmanganese is greater than 15%, problems such as decrease in a corrosionresistance of the steel, increase in difficulties in the manufacturingprocess and increase in manufacturing costs may occur. Also, the workhardenability of the steel may be decreased due to a decreased intensile strength.

Carbon (C) 23%<33.5C—Mn≦37%

Carbon is an element for stabilizing austenite and forming austenite atroom temperature. Carbon increases the strength of the steel.Particularly, carbon dissolved in austenite of the steel increases thework hardenability of the steel and thus increases the wear resistanceof the steel. However, as described above, if the content of carbon inthe steel is insufficient, the stability of austenite is low, and thewear resistance of the steel may be insufficient due to the formation ofmartensite or a low degree of work hardenability of austenite. On theother hand, if the content of carbon in the steel is excessive, it isdifficult to suppress the formation of carbides.

Therefore, in the embodiments of the present disclosure, the content ofcarbon in the steel may be determined according to the contents of otherelements in the steel. The inventors found a relationship between carbonand manganese in the formation of carbides, and the relationship isillustrated in FIG. 1. Although carbides are formed from carbon, theformation of carbides is not affected only by carbon but is affected bya ratio of carbon and manganese. FIG. 1 illustrates a proper content ofcarbon in relation to the content of manganese.

If it is assumed that the contents of the other elements of the steelare within the ranges of the embodiments of the present disclosure, itmay be preferable that the value of 33.5C—Mn be adjusted to be 37 orless (where C and Mn refer to the content of carbon and the content ofmanganese in weight %), so as to prevent the formation of carbides. Thiscorresponds to the right boundary of the parallelogram region in FIG. 1.If 33.5C—Mn is greater than 37, carbides may be formed to a degreeworsening the ductility of the steel. However, if the content of carbonin the steel is too low (that is, if 33.5C—Mn is less than 23), the wearresistance of the steel may not be improved by the work hardenability ofthe steel. Therefore, it may be preferable that 33.5C—Mn be equal to orgreater than 23. That is, it may be preferable that the content ofcarbon satisfy 23<33.5C—Mn≦37

Copper (Cu): 1.6C-1.4(%)≦Cu≦5%

Due to a low solid solubility of copper in carbides and a low diffusionrate of copper in austenite, copper tends to concentrate in interfacesbetween austenite and carbides. Therefore, if fine carbide nuclei areformed, copper may surround the fine carbide nuclei, and thus additionaldiffusion of carbon and growth of carbides may be retarded. That is,copper suppresses the formation and growth of carbides. Therefore, inthe embodiments of the present disclosure, copper is added to the steel.The content of copper in the steel is not independently determined butmay be determined according to the formation behavior of carbides. Forexample, the content of copper may be set to be equal to or greater than1.6C-1.4 weight % so as to effectively suppress the formation ofcarbides if the content of copper in the steel is less than 1.6C-1.4weight %, the conversion of carbon into carbides may not be suppressed.In addition, if the content of copper in the steel is greater than 5weight %, the hot workability of the steel may be lowered. Therefore, itmay be preferable that the upper limit of the content of copper be setto 5 weight %. Particularly, in the embodiments of the presentdisclosure, when the content of carbon added to the steel for improvingwear resistance is considered, the content of copper may preferably be0.3 weight or greater, more preferably, 2 weight % or greater, so as toobtain a sufficient effect of suppressing the formation of carbides.

In the embodiments of the present disclosure, the other component of thesteel is iron (Fe). However, impurities in raw materials ormanufacturing environments may be inevitably included in the steel, andsuch impurities may not be able to be removed from the steel, Suchimpurities are well-known to those of ordinary skill in the art to whichthe present disclosure pertains, and thus descriptions thereof will notbe given in the present disclosure.

In the embodiments of the present disclosure, sulfur (S) and calcium(Ca) may be further included in the steel in addition to theabove-described elements, so as to improve the machinability of thesteel.

Sulfur (S) 0.03% to 0.1%

In general, it is known that sulfur added together with manganese formsmanganese sulfide which is easily cut and separated during a cuttingprocess. That is, sulfur is known as an element improving themachinability of steel. Sulfur is melted by heat generated during acutting process, and thus reduces friction between chips and cuttingtools. That is, sulfur increases the lifespan of cutting tools bylubricating the surface of the cutting tools, reducing the wear on thecutting tools, and preventing accumulation of cutting chips on thecutting tool. However, if the content of sulfur in the steel isexcessive, mechanical characteristics of the steel may deteriorate dueto a large amount of coarse manganese sulfide elongated during a hotworking process, and the hot workability of the steel may deterioratedue to the formation of iron sulfide. Therefore, it may be preferablethat the upper limit of the content of sulfur in the steel be 0.1%. Ifthe content of sulfur in the steel is less than 0.03%, the machinabilityof the steel may not be improved, and thus it may be preferable that thelower limit of the content of sulfur in the steel be 0.03%.

Calcium (Ca) 0.001% to 0.01%

Calcium is usually used to control the formation of manganese sulfide.Since calcium has a high affinity for sulfur, calcium forms calciumsulfide together with sulfur, and along with this, calcium is dissolvedin manganese sulfide. Since manganese sulfide crystallizes aroundcalcium sulfide functioning as crystallization nuclei, during a hotworking process, manganese sulfide may be less elongated and may bemaintained in a spherical shape. Therefore, the machinability of thesteel may be improved. However, if the content of calcium is greaterthan 0.01%, the above-described effect is saturated. In addition, sincethe percentage recovery of calcium is low, a large amount of calcium rawmaterial may have to be used, and thus the manufacturing cost of thesteel may be increased. On the other hand, if the content of calcium inthe steel is less than 0.001%, the above-described effect isinsignificant. Thus, it may be preferable that the lower limit, of thecontent of calcium be 0.001%.

In the embodiments of the present disclosure, chromium (Cr) may beincluded in the steel in addition to the above-described elements so asto further improve the corrosion resistance of the steel.

Cr: 8% or Less (Excluding 0%)

Generally, manganese lowers the corrosion resistance of steel. That is,in the embodiments of the present disclosure, manganese included in thesteel in the above-described content range may lower the corrosionresistance of the steel, and thus chromium is added to the steel toimprove the corrosion resistance of the steel in addition, if chromiumis added to the steel in an amount within the range, the strength of thesteel may also be improved. However, if the content of chromium in thesteel is greater than 8 weight %, the manufacturing cost of the steel isincreased, and carbon dissolved in the steel may be converted intocarbides along grain boundaries to lower the ductility of the steel andparticularly resistance of the steel to sulfide stress cracking. Inaddition, ferrite may be formed in the steel, and thus austenite may notbe formed as a main microstructure in the steel. Therefore, it may bepreferable that the upper limit of the content of chromium be 8 weight%. Particularly, to maximize the effect of improving the corrosionresistance of the steel, it may be preferable that the content ofchromium in the steel be set to be 2 weight % or greater. Since thecorrosion resistance of the steel is improved by the addition ofchromium, the steel may be used for forming slurry pipes or as an antisour gas material.

The steel having the above-described composition is austenitic steelhaving 90 area % or more of austenite. In a later processing process,austenite of the steel may be markedly hardened, and thus the steel mayhave a high degree of hardness. In addition to austenite, some othermicrostructures such as martensite, bainite, pearlite, and ferrite maybe inevitably formed in the steel as impurity microstructures. In thepresent disclosure, the sum of the amounts of the phases of the steel isput as 100%, and the content of each microstructure is denoted as aproportion of the sum without considering the amounts of precipitatessuch as a carbide precipitate.

Furthermore, in the embodiments of the present disclosure, it may bepreferable that the steel include 10 area % or less of carbides (basedon the total area of the steel). Since carbides lower the ductility ofthe steel, the amounts of carbides in the steel may be adjusted to below. For example, in the embodiments of the present disclosure, sincethe area fraction of carbides in the steel is 10% or less, when thesteel is used as wear resistant steel, problems caused by low ductilitysuch as premature fracturing and a decrease in impact toughness may notarise.

Hereinafter, a method for producing the wear resistant austenitic steelwill be described according to an embodiment of the present disclosure.The steel may be produced by a manufacturing method commonly known inthe related art, and the manufacturing method of the related art mayinclude a conventional hot rolling process in which a slab is reheated,roughly-rolled, and finish-rolled. After the hot rolling process, thesteel may be cooled by a conventional cooling method. For example, in anembodiment of the present disclosure, the steel may be produced by anexemplary method proposed by the inventors as follows.

A steel slab is prepared, which includes, by weight %, 8% to 15% ofmanganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu)satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) andinevitable impurities.

As described above, the steel slab may further include sulfur (S) andcalcium (Ca).

Furthermore, as described above, the steel slab may further includechromium (Cr).

The steel slab is reheated to a temperature of 1050° C. to 1250° C.

The steel slab (or ingot) may be reheated in a reheating furnace for ahot rolling process. If the steel slab is reheated to a temperaturelower than 1050° C., the load acting on a rolling mill may be markedlyincreased, and alloying elements may not be sufficiently dissolved inthe steel slab. On the other hand, if the reheating temperature of thesteel slab is too high, crystal grains may excessively grow, and thusthe strength of the steel slab may be lowered. Particularly, in theabove-described composition range of the steel of the presentdisclosure, carbides may melt in grain boundaries, and if the steel slabis reheated to a temperature equal to or higher than the solidus line ofthe steel slab, hot-rolling characteristics of the steel slab maydeteriorate. Therefore, the upper limit of the reheating temperature maybe set to be 1250° C.

Thereafter, the steel slab is finish-rolled at a temperature of 800° C.to 1050° C. to form a steel sheet.

As described above, the steel slab is rolled within the temperaturerange of 800° C. to 1050° C. If the steel slab is roiled at atemperature lower than 800° C., the load of rolling may be large, andcarbides may precipitate and grow coarsely. Thus, desired ductility maynot be obtained. The upper limit of the rolling temperature is set to be1050° C.

The steel sheet formed by hot rolling is cooled to a temperature of 600°C. or lower at a cooling rate of 10° C./s to 100° C./s.

After the finish rolling, the steel sheet may be cooled at asufficiently high cooling rate to suppress the formation of carbides ingrain boundaries. If the cooling rate is less than 10° C./s, theformation of carbides may not be sufficiently suppressed, and thuscarbides may precipitate in grain boundaries during cooling. This maycause problems such as premature fracture, a ductility decrease, and awear resistance decrease. Therefore, the cooling rate may be adjusted tobe high, and the upper limit, of the cooling rate may not be limited toa particular value as long as the cooling rate is within an acceleratedcooling rate range. However, it may be difficult to increase the coolingrate to a value greater than 100° C./s by a conventional acceleratedcooling technique.

Although the steel sheet is cooled at a high cooling rate, if thecooling of the steel sheet is terminated at a high temperature, carbidesmay be formed and grow in the steel sheet. Therefore, in the embodimentof the present disclosure, the steel sheet may be cooled to atemperature of 600° C. or lower.

MODE FOR INVENTION

Hereinafter, the embodiments of the present disclosure will be describedmore specifically through examples. However, the examples are forclearly explaining the embodiments of the present disclosure and are notintended to limit the spirit and scope of the present disclosure.

Example 1

Slab samples having elements and compositions illustrated in Table 1were reheated, hot-rolled, and cooled under the conditions illustratedin Table 2. Then, properties of the samples such the microstructure,elongation, strength, and carbide fraction were measured as illustratedin Table 3. In Table 1, the content of each element is given in weight.

TABLE 1 No. C Mn Cu Cr 33.5C—Mn 1.6C-1.4 Comparative 0.5 10 6.8 — sampleA1 Comparative 1.2 10 30.2 0.5 sample A2 Comparative 1.45 12 0.75 36.60.9 sample A3 Comparative 1.3 12 0.52 31.6 0.7 sample A4 Comparative1.23 14.1 1.05 1.98 27.1 0.6 sample A5 Inventive 1 9 1.2 24.5 0.2 sampleA1 Inventive 1.2 15 1 0.5 25.2 0.5 sample A2 Inventive 1.03 10 0.55 0.524.5 0.2 sample A3 Inventive 1.4 15 1.6 1.1 31.9 0.8 sample A4 Inventive1.25 14 1.02 2 27.9 0.6 sample A5 Inventive 1.15 14.6 0.87 3 23.9 0.4sample A6

TABLE 2 Reheating Finish rolling Cooling temperature temperature rateCooling stopping No. (° C.) (° C.) (° C./s) temperature (° C.)Comparative 1160 895 13 550 sample A1 Comparative 1140 930 8 561 sampleA2 Comparative 1140 924 21 568 sample A3 Comparative 1140 921 16 485sample A4 Comparative 1145 915 5.6 545 sample A5 Inventive 1145 915 15561 sample A1 Inventive 1142 889 15 512 sample A2 Inventive 1152 875 17579 sample A3 Inventive 1140 906 25 532 sample A4 Inventive 1146 911 25541 sample A5 Inventive 1143 892 20 521 sample A6

TABLE 3 Austenite Carbide Yield Tensile fraction fraction Elongationstrength strength No. (area %) (area %) (%) (MPa) (MPa) Comparative 63<1 7.8 340 590 sample A1 Comparative 87 13 4.6 415 669 sample A2Comparative 88 12 3.7 572 865 sample A3 Comparative 89 11 4.4 452 721sample A4 Comparative 87.6 12.4 8.2 452 765 sample A5 Inventive 98 2 37398 982 sample A1 Inventive 99 1 43 420 1012 sample A2 Inventive 99 1 35406 964 sample A3 Inventive 99 1 40 542 1108 sample A4 Inventive 99 1 42462 976 sample A5 Inventive 99 1 43 572 1095 sample A6

In addition, a wear test (ASTM G65) and an immersion test (ASTM G31) forevaluating corrosion rates were performed on comparative samples andinventive samples, and the results are illustrated in Table 4 below.

TABLE 4 Weight Corrosion rate (mm/year) reduction 3.5% NaCl, 50° C., No.(g) 2 weeks 0.05M H₂SO₄, 2 weeks Comparative 0.72 0.14 0.48 sample A1Comparative 0.36 0.15 0.49 sample A2 Comparative 0.24 0.17 0.52 sampleA3 Comparative 0.29 0.16 0.50 sample A4 Inventive 0.35 0.14 0.48 sampleA1 Inventive 0.28 0.17 0.50 sample A2 Inventive 0.34 0.16 0.49 sample A3Inventive 0.18 0.17 0.50 sample A4 Inventive 0.31 0.09 0.41 sample A5Inventive 0.27 0.07 0.37 sample A6

33.5C—Mn of Comparative Sample A1 was 6.8 which was outside of the rangeof the embodiments of the present disclosure. Thus, due to a lack ofcarbon stabilizing austenite, a large amount of martensite was formed inComparative Sample A1, and a desired austenitic microstructure was notformed in Comparative Sample A1.

Comparative Sample A2 had manganese and carbon within the content rangesof the embodiments of the present disclosure. However, copper was notadded to Comparative Sample A2, and thus the formation of carbides wasnot suppressed. That is, large amounts of carbides were formed alonggrain boundaries of Comparative Sample A2, and thus a desiredmicrostructure and elongation were not obtained. In Comparative SampleA2, a sufficient degree of work hardenabiliy was not obtained due topremature fracture and a decreased amount of dissolved carbon caused bythe formation of carbides. Therefore, the wear amount of ComparativeSample A2 was relatively large.

Comparative Samples A3 and A4 had manganese and carbon within thecontent ranges of the embodiments of the present disclosure. However,the content of copper in each of Comparative Samples A3 and A4 wasoutside of the range of the embodiments of the present disclosure.Therefore, like in Comparative Sample A2, large amounts of carbides wereformed in Comparative Samples A3 and A4, and thus a desiredmicrostructure and elongation were not obtained. Since the contents ofcopper in Comparative Samples A3 and A4 were outside of the range of theembodiments of the present disclosure, the formation of carbides was noteffectively suppressed, and thus the amounts of dissolved carbon andelongation of Comparative Samples A3 and A4 were reduced to causepremature fracture. Thus, a sufficient degree of work hardenability wasnot obtained in Comparative Sample A3 and A4, and thus the wearresistance of Comparative Samples A3 and A4 was reduced.

Although the composition of Comparative Sample A5 satisfied theconditions of the embodiments of the present disclosure, the coolingrate of Comparative Sample A5 after rolling process was outside of therange of the embodiments of the present disclosure. That is, due to alow cooling rate, the formation of carbides was not effectivelysuppressed, and thus the ductility of Comparative Sample A5 wasdecreased.

However, in Inventive Samples A1 to A6 having elements and compositionsaccording to the embodiments of the present disclosure, the formation ofcarbides in grain boundaries was effectively suppressed owing to theaddition of copper, and thus physical properties of Inventive Samples A1to A6 were not worsened. In detail, although Inventive Samples A1 to A6had high carbon contents, the formation of carbides was effectivelysuppressed owing to the addition of copper, and thus Inventive SamplesA1 and A6 had desired microstructures and properties. Since carbon wassufficiently dissolved in austenite and the formation of carbides ingrain boundaries was effectively suppressed, the elongation of InventiveSamples A1 to A6 was stably maintained, and the tensile strength ofinventive Samples A1 to A6 was high. Therefore, the work hardenabilityof Inventive Samples A1 to A6 was sufficient, and thus the wear amountsof Inventive Samples A1 to A6 were small.

Particularly, according to results of a corrosion test, the corrosionrates of inventive Samples A5 and A6 to which chromium was additionallyadded were low. That is, the corrosion resistance of Inventive SamplesA5 and A6 was improved. The effect of improving corrosion resistance bythe addition of chromium may be clearly understood by comparison withcorrosion rates of Inventive Samples A1 to A4. In addition, the strengthof Inventive Samples A5 and A6 was improved by solid-solutionstrengthening induced by the addition of chromium.

FIG. 2 is a microstructure image of inventive Sample A2. Referring toFIG. 2, although inventive Sample A2 has a high carbon content, carbidesare not present in inventive Sample A2 owing to the addition of copperwithin the content range of the embodiments of the present disclosure.

Example 2

Steel slabs (Inventive Samples and Comparative Samples) havingcompositions illustrated in Table 5 were manufactured by a continuouscasting process. In Table 5, the content of each element is given inweight %.

TABLE 5 No. C Mn Cu Cr Ca S 33.5C—Mn 1.6C-1.4 Comparative 1 9 1.2 24.50.2 sample B1 Comparative 1.2 15 1 0.5 0.02 25.2 0.5 sample B2Comparative 1.03 10 0.55 0.5 24.5 0.2 sample B3 Comparative 1.4 15 1.61.1 0.01 31.9 0.8 sample B4 Comparative 1.25 14 1.02 2 27.9 0.6 sample35 Inventive 0.98 9.2 1.5 0.006 0.06 23.6 0.2 sample B1 Inventive 1.029.8 0.53 0.48 0.007 0.05 24.4 0.2 sample B2 Inventive 1.04 10.5 0.530.45 0.007 0.07 24.3 0.3 sample B3 Inventive 0.98 10.6 0.57 0.53 0.0080.09 22.2 0.2 sample B4 Inventive 1.23 14.8 1.11 1.95 0.006 0.08 26.40.6 sample B5

The steel slabs were reheated, finish-rolled, and cooled under theconditions illustrated in Table 6 so as to form steel sheets.

TABLE 6 Finish Cooling Reheating rolling Cooling stopping temperaturetemperature rate temperature No. (° C.) (° C.) (° C./s) (° C.)Comparative 1145 915 15 561 sample B1 Comparative 1142 889 15 512 sampleB2 Comparative 1152 875 17 579 sample B3 Comparative 1140 906 25 532sample B4 Comparative 1146 911 25 541 sample B5 Inventive 1142 889 15552 sample B1 Inventive 1152 875 17 579 sample B2 Inventive 1150 890 19580 sample B3 Inventive 1146 886 19 575 sample B4 Inventive 1143 892 24541 sample B5

The austenite fraction, carbide fraction, elongation, yield strength,and tensile strength of each of the steel sheets were measured asillustrated in Table 7. Holes were repeatedly formed in each of thesteel sheets by using a drill having a diameter of 10 mm and formed ofhigh speed tool steel in conditions of a drill speed of 130 rpm and adrill movement rate of 0.08 mm/rev. The number of holes formed in eachsteel sheet until the drill was worn down to the end of its lifespan wascounted as illustrated in Table 3.

TABLE 7 Austenite Carbide Yield Tensile fraction fraction Elongationstrength strength Number No. (area %) (area %) (%) (MPa) (MPa) of holesComparative 98 2 37 398 982 1 sample B1 Comparative 99 1 43 420 1012 0sample B2 Comparative 99 1 35 406 964 1 sample B3 Comparative 99 1 40542 1108 0 sample B4 Comparative 98 1 42 462 976 0 sample B5 Inventive99 1 36 386 991 3 sample B1 Inventive 99 1 36 410 360 4 sample B2Inventive 99 1 34 405 953 5 sample B3 Inventive 99 1 35 408 955 6 sampleB4 Inventive 99 1 41 461 984 3 sample 35

In addition, a wear test (ASTM G65) and an immersion test (ASTM G31) forevaluating corrosion rates were performed on each of the steel sheets(comparative samples and inventive samples), and the results areillustrated in Table 8 below.

TABLE 8 Wear test Weight Corrosion rate (mm/year) reduction 3.5% NaCl,50° C. No. (g) 2 weeks 0.05M H₂SO₄, 2 weeks Comparative 0.35 0.14 0.48sample B1 Comparative 0.28 0.17 0.50 sample B2 Comparative 0.34 0.160.49 sample B3 Comparative 0.18 0.17 0.50 sample B4 Comparative 0.310.09 0.41 sample B5 Inventive 0.34 0.15 0.50 sample B1 Inventive 0.340.16 0.48 sample B2 Inventive 0.33 0.17 0.50 sample B3 Inventive 0.320.16 0.47 sample B4 Inventive 0.30 0.09 0.40 sample B5

In the inventive samples having carbon and manganese within the contentranges of the embodiments of the present disclosure, the formation ofcarbides in grain boundaries was effectively suppressed owing to theaddition of copper, and thus physical properties of the inventivesamples were not worsened. In detail, although the inventive samples hadhigh carbon contents, the formation of carbides was effectivelysuppressed owing to the addition of copper, and thus the inventivesamples had desired microstructures and properties. Since carbon wassufficiently dissolved in austenite and the formation of carbides ingrain boundaries was effectively suppressed, the elongation of theinventive samples was stably maintained, and the tensile strength of theinventive samples was high. Therefore, the work hardenability of theinventive samples was sufficient, and thus the wear amounts of theinventive samples were small.

The machinability of Comparative Samples B1 to B5 was poor becausesulfur and calcium were not added to Comparative Samples B1 to B5 or thecontents of sulfur and calcium in Comparative Samples B1 to B5 wereoutside of the ranges of the embodiments of the present disclosure.

However, Inventive Samples B1 to B5 including sulfur and calcium withinthe content ranges of the embodiments of the present disclosure hadsuperior machinability as compared with the comparative samples.Particularly, in Inventive Samples B2 to B4 having different sulfurcontents, the machinability thereof was improved in proportion to thecontent of sulfur.

FIG. 3 illustrates machinability with respect to the content of sulfur.Referring to FIG. 3, machinability improves in proportion to the contentof sulfur.

1. Wear resistant austenitic steel having superior machinability andtoughness in weld heat affected zones thereof, the wear resistantaustenitic steel comprising, by weight %, manganese (Mn): 15% to 25%,carbon (C): 0.8% to 1.8%, copper (Cu) satisfying 0.7C-0.56(%)≦Cu≦5%, andthe balance of iron (Fe) and inevitable impurities, wherein the weldheat affected zones have a Charpy impact value of 100 J or greater at−40° C.
 2. The wear resistant austenitic steel of claim 1, furthercomprising, by weight %, sulfur (S): 0.03% to 0.1%, and calcium (Ca):0.001% to 0.01%.
 3. The wear resistant austenitic steel of claim 1,further comprising, by weight %, chromium (Cr): 8% or less (excluding0%), wherein the wear resistant austenitic steel has a yield strength of450 MPa or greater.
 4. The wear resistant austenitic steel of claim 1,wherein the weld heat affected zones have a microstructure comprising 95volume % or more of austenite.
 5. The wear resistant austenitic steel ofclaim 1, wherein the weld heat affected zones have a microstructurecomprising 5 volume % or less of carbides.
 6. A method of producing wearresistant austenitic steel having superior machinability and toughnessin weld heat affected zones thereof, the method comprising: reheating asteel slab to a temperature of 1050° C. to 1250° C., the steel slabcomprising, by weight %, manganese (Mn): 15% to 25%, carbon (C): 0.8% to1.8%, copper (Cu) satisfying 0.7C-0.56(%)≦Cu≦5% where C denotes acontent of the carbon (C) by weight %, and the balance of iron (Fe) andinevitable impurities; and performing a finish rolling process on thereheated steel slab within a temperature range of 300° C. to 1050° C. 7.The method of claim 6, wherein the steel slab further comprises, byweight %, sulfur (S): 0.03% to 0.1%, and calcium (Ca): 0.001% to 0.01%.8. The method of claim 6, wherein the steel slab further comprises, byweight %, chromium (Cr): 8% or less (excluding 0%), and the steel slabhas a yield strength of 450 MPa or greater.